The present invention relates generally to compositions which are added to liquid samples being tested for infrared absorbence, and more particularly to a preservative composition which can be added to milk samples and which exhibits a substantially equal increased in infrared absorbence measured at selected pairs of wavelengths.
It is common practice in the dairy industry to send samples of various dairy products to centralized laboratories for testing. Typical measurements include the content of fat, protein, lactose, total solids, and added water. The test samples can be in transit for one or more days between the dairy farm and the central testing laboratory and, even when refrigerated, are subject to bacterial degradation. In order to prevent such degradation (which can affect the composition of the sample as well as foul the test equipment), preservatives are routinely added to the milk sample before leaving the dairy farm.
At the central testing laboratory, the milk sample composition is typically measured based on the infrared absorbence of the sample. The consituents which are to be measured exhibit characteristic absorbencies at different wavelengths, allowing determination of the concentration of each constituent based on the differencce between absorbencies observed at two different wavelengths. In particular, the present invention is concerned with the "double wavelength" determination of milk composition as described in U.S. Pat. Nos. 4,247,773 and 4,236,075 to Nexo et al. That method, which applies broadly to the quantitative measurement of a constituent (dispersed phase) of an emulsion or suspension, relies on the measurement of infrared absorbence at two predetermined wavelengths for each of the constituents to be measured. The first (sample) wavelength is chosen at a frequency where the constituent exhibits a high level of absorbency. The second (reference) wavelength is a frequency where the constituent displays a minimum infrared absorbency. The content of the particular constituent can then be estimated based on the difference between the absorbencies. The wavelength pairs which correspond to the constituent measured will be referred to as "channels" hereinafter.
The accuracy of such a measurement system is limited by cross interference of other components present in the milk. Even in fresh milk, the principle constituents (i.e. fat, protein, and lactose) absorb infrared light at all wavelengths so that the light absorption at any particular wavelength is influenced, to some extent, by all of the constituents present. While this effect can be minimized by selecting wavelengths at which the constituent of interest absorbs strongly and the other constituents absorb less strongly, cross interference cannot be completely eliminated. The analyzer described in U.S Pat. No. 4,236,075, partially compensates for such cross interference by subtracting out the estimated effect of the other constituents based on its initial (uncorrected) measurement. In the example of milk, the lactose absorbency is reduced by the estimated contribution of protein and fat at the lactose wavelength based upon the initial measurements of protein and fat. The measurements of the other constituents are similarly adjusted.
While such iterative adjustment of the measurements works reasonably well to compensate for variations in the concentrations of the principle constituents of an emulsion, it is not able to correct for variations in trace constituents which are not being measured. In particular, the analyzer of U.S. Pat. No. 4,236,075 and similar double wavelength analyzers, are unable to compensate for components which are added to the milk sample, such as the components in a preservative composition. For that reason, the addition of such preservative compositions leads to erroneous measurements. For example, addition of the preservative can affect the measurement of fat, lactose, and protein by as much as 5 to 10% of the actual value.
It would thus be desirable to provide a preservative composition which, when added to fresh milk, will not affect the double wavelength infrared measurement of the milk composition.